(1) Field of the Invention
The present invention relates to an optical apparatus for bidirectional optical communication, which allocates different wavelengths to upstream (transmission) and downstream (reception) optical signals, to transmit each optical signal using one optical fiber, in optical communication.
(2) Related Art
In recent years, as integrated transmission and reception type optical transceivers have become widespread, also in communication systems, attention has been given to bidirectional optical communication wherein transmission and reception are performed simultaneously using just one optical fiber. For example, in the case where bidirectional optical transmission is performed between a local station and a remote station, by allocating different wavelengths for transmission (from the local station to the remote station) and reception (from the remote station to the local station) by using WDM (Wavelength Division Multiplexing) techniques, the transmission and reception can be performed using the same optical transmission path. In such bidirectional optical communication, since it is possible to reduce the number of optical transmission paths, there is an effect that the cost required for the laying, maintenance and administration of the optical fiber can be reduced. Especially, from the standpoint of constructing a simple and economical WDM network, there is expectation of bidirectional optical communication using CWDM (Coarse Wavelength Division Multiplexing) techniques with wide wavelength spacing. Therefore, also in the optical apparatus for bidirectional optical communication, there is a problem of realizing an apparatus with low cost and high functionality.
Incidentally, in optical apparatuses for bidirectional optical communication, in the case where optical processing, for example, optical spectrum supervision, optical amplification, dispersion compensation and the like, is performed on a transmitted light and a received light, it is generally difficult to make means for realizing such optical processing to be common to both the transmission side and the reception side. For example, in a known optical fiber amplifier and the like, usually, since an optical isolator is disposed on the optical path in order to reduce an influence by the reflection of pumping light, it is possible to cope with only optical signals in a single direction. In the case where the optical isolator is omitted from such an optical fiber amplifier in order to cope with bidirectional optical signals, there is a possibility that coherent cross talk occurs between signal wavelengths, resulting in deterioration of a transmission characteristic. Therefore, in an optical apparatus such as a conventional optical fiber amplifier or the like, as shown in FIG. 8 for example, it is necessary to separate optical signals being propagated bidirectionally through a transmission path fiber, into the optical signals in each one-way direction using optical path direction selecting means such as an optical circulator or the like, and provide an individual optical processing section for each of the optical signals. Accordingly, in an optical apparatus with a configuration as described above, there is a problem in that there is no merit by bidirectional optical communication in terms of the cost, power consumption and the like.
Note, the expression “optical processing” used in the present specification means processing such as the abovementioned optical spectrum supervision, optical amplification, dispersion compensation and the like, and does not refer to the contents of calculations or the like using optical signals.
As a technique for solving the problems described above, an apparatus as shown in FIG. 9, for example, has been proposed (refer to Japanese Unexamined Patent Publication No. 11-127121) in which the steams of the optical signals separated into each one-way direction by optical circulators, are made into the steam in a single direction using a pair of optical multiplexing/demultiplexing devices, so that bidirectional optical signals can be processed by one optical processing section.
However, in the conventional configuration as shown in FIG. 9, by making the steams of bidirectional optical signals into the steam in a single direction, it is no longer necessary to individually provide optical processing sections corresponding to each of the directions. However, the structure around the optical processing section is complicated, so there is a disadvantage in that the number of components is increased. Such an increase in the number of components is not only accompanied by extra loss in the optical signals, but it also decreases the benefits of cost reduction in the whole apparatus. The integration of optical components for example is effective for reducing optical signal loss and reducing the apparatus cost. However, since all of the optical components disposed around the optical processing section, such as optical circulators, optical multiplexing/demultiplexing devices and the like, are normally constructed using different devices, it is difficult to integrate such optical components.